Glyphosate resistance

Facts About Glyphosate-Resistant Weeds is produced by the Glyphosate Stewardship Working Group, a coalition of university weed scientists. More publications are at their website Glyphostate, Crops, and Weeds.

From: Facts About Glyphosate-Resistant Weeds, Chris Boerboom, University of Wisconsin, and Michael Owen, Iowa State University.

With the increasing reports of glyphosate-resistant weeds, many growers want to know if resistant weeds are likely to develop in their fields. There is not a simple answer, but there are certain risk factors. A weed’s potential for developing glyphosate resistance is primarily determined by three factors: weed biology, intensity of glyphosate use, and glyphosate rate.

Weed biology

It is likely that certain weed species have greater genetic diversity, so there is a greater risk that they will develop herbicide resistance. Weed species that
have already developed resistance to other herbicides may have a greater
probability of developing glyphosate resistance. Species that may be prone to
glyphosate resistance based on resistance to other herbicide modes of action
include pigweed species (including waterhemp, and Palmer amaranth), common lambsquarters, common and giant ragweed, kochia, and ryegrass. Since the trait for glyphosate resistance can spread by pollen or seed, the spread of resistant populations will be faster for some weed species than others.

 

Intensity of glyphosate use increases probability of resistance

Increasing the intensity of glyphosate use (frequency and number of acres treated) increases the probability of selecting an herbicide-resistant plant. With continued glyphosate use, the number of resistant plants will continue to multiply and create a resistant population. Herbicides do not cause the mutations that result in resistance. Rather, an extremely rare genetic trait that allows a weed to survive glyphosate may exist in the natural population. It is more likely to be found and increase when glyphosate is used frequently.

 

Glyphosate rate

It is less clear how rate affects glyphosate resistance development in weeds. Several known glyphosate-resistant weeds require eight to 10 times more glyphosate to be controlled than the normal, sensitive biotypes. This level of resistance means that labeled glyphosate rates will not control these weeds, and that making applications at labeled rates probably will not prevent resistance. There are other cases of glyphosate-resistant weeds that appear to have resistance at two to four times greater than normal biotypes. It is unknown if reduced glyphosate rates contributed to these plants becoming resistant, or if resistance would have occurred at labeled rates. Regardless, it is wise to use glyphosate at labeled rates to ensure good control and prevent hard-to-control weeds from increasing in any field.

 

Not all weed escapes are due to glyphosate resistance

The majority of weed escapes in Roundup Ready® fields are not due to glyphosate resistance. Rather, some escapes can be attributed to certain weed species that are naturally tolerant to glyphosate. Examples include yellow nutsedge, field horsetail, morningglories, prairie cupgrass, wild buckwheat, and dayflower species. Labeled glyphosate rates are less effective against these naturally tolerant species. Also, environmental and application factors can lead to glyphosate performance problems. Factors that may reduce glyphosate performance include:

• Incorrect rate for weed size or species.
• Rain before glyphosate is completely absorbed by the weed.
• Weather-stressed weeds (from drought, cold, etc.).
• Incomplete spray coverage of weeds below the canopy.
• Reduced glyphosate activity with early morning, late evening, or night applications.
• Weeds emerging after the glyphosate application.

Prevention is the key

Adding diversity to your weed management programs is the key to avoiding resistance development.A more integrated weed management program provide additional benefits since these practices improve the overall level and consistency of weed control. The following weed management practices avoid the continuous and exclusive use of glyphosate and lessen the potential for developing glyphosate-resistant weeds:

• Rotate between Roundup Ready® and conventional crops or crops with other types of herbicide resistance. Use Roundup Ready® crops and glyphosate in your crop rotation where they have the greatest economic and management value.
• Rotate glyphosate with herbicides that have different modes of action.
• Apply a residual herbicide before glyphosate or tank mix another herbicide with glyphosate.
• Avoid making more than two glyphosate applications to a field over a two-year period.
• If glyphosate is used as a burndown treatment and in-crop in the same year, tank mix the glyphosate applied in the burndown treatment with an herbicide that has a different mode of action. The in-crop glyphosate application should still be rotated with other herbicides in other years.
• Use cultivation and other mechanical weed management practices. In addition, apply glyphosate at labeled rates and at the correct stage of weed and crop growth to reduce the risk of poor control.
• Scout fields regularly, identify the weeds present, and record their locations on maps to allow a quick response to changes in weed populations.
  

Resources

The information on this page is from Facts About Glyphosate-Resistant Weeds, the first in a series of publications written by members of the Glyphosate Stewardship Working Group, a coalition of university weed scientists, and published by Purdue University. Their website is http://www.glyphosateweedscrops.org/.

Other publications in this series are:

Biology and Management of Giant Ragweed
Biology and Management of Common Lambsquarters
Understanding Glyphosate To Increase Performance
Biology and Management of Horseweed
Biology and Management of Wild Buckwheat
Corn and Soybean Herbicide Mode of Action Chart
Biology and Management of Common Waterhemp

You can view the publications online in pdf format at the Glyphosate, Crops, and Weeds website, or obtain print copies from the Purdue University Media Store.